The present invention relates to a method and system for magnetic resonance imaging (MRI) and, more particularly, to the method and system in which a suppression effect of magnetic resonance (MR) signals from fat within an object being diagnosed is improved by using a pulse sequence including a procedure of applying a prepulse before scan for imaging or exciting selectively protons of water within the object.
In the MR image acquisition, there is a case in which obtaining a so-called MTC (Magnetization Transfer Contrast) is significant from a clinical point of view.
FIG. 1 illustrates the spectrum of protons contained in water, fat and high polymer. As can be seen in FIG. 1, protons contained in water have a resonance frequency of about 64 MHz under a magnetic field of, for example, 1.5 tesla. However, the resonance frequency of protons contained in fat, as well known, shifts toward the low frequency side (the right side in the figure) by 3.5 PPM by chemical shift, and under the above-mentioned magnetic field shifts by about 224 Hz. Meanwhile, protons contained in high polymer have characteristics such that their frequency width is extremely wide.
If a frequency offset by, for example, 500 Hz from the resonance frequency of protons of water is selectively excited before an ordinary imaging sequence is performed, the level of MR signals from the protons of high polymer decreases as indicated by the broken line. Further, the level of the MR signals from the protons contained in water decreases as indicated by the broken line. This is thought to be due to the fact that the protons of water are cross relaxed or exchanged with the protons of high polymer, which fact is already known as an MTC (Magnetization Transfer Contrast) effect.
Use of such an MTC effect makes it possible to obtain an image having a contrast different from that of the prior art in proportion to the ratio at which the high polymer is present. Also, since the level of signals of the parenchyma portion can be advantageously reduced much more than the level of signals of the blood vessel portions, the MTC effect is applied to angiography in which fine blood vessels are extracted.
FIG. 2 illustrates an example of a pulse for selectively exciting protons of high polymer. The pulse shown in this figure is what is commonly called a binomial pulse formed of a plurality of pulse groups such that the pulse length is set at a ratio represented by a binomial distribution and its polarity is alternately reversed. Particularly, the binomial pulse shown therein is such that the pulse length is set at 1:2:1 (hereinafter such a binomial pulse will be abbreviated as a 121 pulse), and the time .tau. between two pulses is short, thus the total time being set at approximately 1 msec by taking the above-mentioned frequency into consideration.
FIG. 3 is a curve representing the magnetization characteristics of the 121 pulse. The vertical axis depicts magnetization Mz, and the horizontal axis depicts the amount of shift from the central frequency f.sub.0. This curve shows the following: protons having a spectrum value near the central frequency are not excited, thus no influences are left in an imaging sequence performed after the binomial pulse is applied. However, protons having a spectrum value which is apart from the central frequency by 500 Hz or more are excited by the binomial pulse, no magnetization is left, and the level of MR signals obtained in a later imaging sequence is reduced. That is, the range of frequencies at which signals can be extracted in a later imaging sequence corresponds to the top portion of the curve of the figure, and the range of frequencies at which signals cannot be extracted corresponds to the bottom portion of the curve of the figure.
Therefore, when the central frequency f.sub.0 of such a binomial pulse is applied where the frequency is made to match the resonance frequency of protons of water, the protons of water are not excited. However, the protons of high polymer are excited in a bottom portion of 500 Hz or above. Thus, it is possible to decrease the level of the MR signals from the high polymer in the imaging sequence performed subsequently to the above. By selectively exciting protons of high polymer beforehand by using a binomial pulse as described above, it is possible to obtain the above-described MTC effect.
Meanwhile, in the MR image acquisition, as fat which is said to have a small clinical significance does not appear in the image, a so-called imaging method capable of suppressing MR signals from fat is often used. There are a number of imaging methods in which MR signals coming from fat is suppressed. In one of them, a method of using a binomial pulse capable of exciting protons in a frequency selecting manner is known.
Since, as described above, the level of signals which appear in a later imaging sequence decreases in the bottom portion of magnetization Mz of the binomial pulse, the binomial pulse is set in such a way that the resonance frequency of protons contained in fat approaches the bottom portion.
FIG. 4 shows a 1331 pulse set in this manner. The time .tau. between the pulses is set at approximately 2.3 msec (longer) so that the resonance frequency of protons contained in water approaches the central frequency f.sub.0 and the resonance frequency of protons contained in fat approaches the bottom portion offset toward the low frequency side by approximately 220 Hz from the central frequency f.sub.0 of the protons contained in fat.
If such a binomial pulse is applied and then a normal imaging sequence is performed, it is possible to form only protons of free water into an image and suppress the level of MR signals from the protons contained in fat.
On one hand, in an MRI system, uniformity of a static magnetic field becomes a key factor to increase quality of an excitation spectrum (i.e., signal to noise ratio and resolution). Thus MRI systems are generally provided with an adjusting means with a shim coil in order to adjust the uniformity of the field. Particularly in medical use, it is preferred to prevent a patient from being placed in the diagnostic space of a magnet for a long time. Therefore, the first-order shimming (gradient shimming) in X-, Y-, and Z-directions, which is simply operated, is carried out to separate resonance frequencies of water and fat in a spectrum.
However, since the length of the entire pulse becomes as much as approximately 7 msec in the 1331 pulse, this makes a repetition time TR longer. In other words, this pulse has the problem that it is difficult to use the signal for an ultra-fast scan and angiography in which the repetition time TR is made short and used.
If, in contrast, the pulse interval .tau. is made short for decreasing the repetition time TR, another problem occurs, for example, the top portion of the graph of magnetization Mz is expanded and fat is also formed into an image, decreasing the suppression effect of MR signals from fat.
Further, the prepulse is liable to be influenced by ununiformity of the static magnetic field, thus decreasing quality of images. Furthermore, since a frequency range distant from the resonance of water is excited (that is, off-resonance), the MTC effect will also be occurred, decreasing a signal to noise ratio correspondingly to it.
On one hand, there is a drawback with respect to the aforementioned first-order shimming. It is impossible for the first-order shimming to achieve complete uniformity of the static magnetic field; there will be still turbulence in higher-order magnetic components of the static magnetic field. This ununiformity of high-order magnetic components frequently cause the curve of water to deviate from the central frequency f.sub.0 of a prepulse (for instance, a binomial pulse of 1331).
FIG. 5 exemplifies a change in frequency as a slicing position changes, which is caused by the ununiformity of the higher-order magnetic components. The degree of deviation of a certain frequency f=f.sub.1 becomes noticeable as the slicing position is distant from the center in the slicing direction. In case that a prepulse is used in multi-slice imaging in order to suppress MR signals from fat, the aforementioned first-order shimming is effective in making the resonance curve of water coincide with the central frequency f.sub.0, as shown in FIG. 6A, for the central slicing plane in the slicing direction, the central slicing plane usually being coincident with an iso-center. However, at slicing positions moved from the center in the slicing direction, the above-said higher-order magnetic components may cause the resonance curve of water to deviate from the central frequency f.sub.0 of a prepulse, as shown in FIG. 6B, with the result that the resonance curves of water and fat move out of the top and bottom portions of the prepulse, respectively. In consequence, when the multi-slice imaging is carried out, there is a problem that image quality alters with changed slicing positions, because slicing planes at the central position and its neighboring positions in the slicing direction have a great suppressing effect of MR signals from fat, while other slicing planes distant from the central slicing position have a less suppressing effect of it.